The thing is, I'd think the plan -- at this point, anyway -- would be to groom Vrabel for DC.

If there's a plan (if I were Crennel) I'd be wanting to "groom" Vrabel & Johnson for HC..... I doubt they have any loyalty for the Texans & though preparing Vrabel/Johnson for HC position may benefit the Texans in the near term, the ultimate goal is to make Vrabel & Johnson HCs.... if they have the aptitude for it.